The present invention relates to the field of Photo-Voltaic (PV) solar power, and in particular to methods and apparatuses for providing a safety short mechanism for a solar cell group.
PV solar modules generate electricity responsive to solar light energy, and are typically arranged in a series loop with a DC/AC converter, the DC/AC converter arranged to convert the DC power of the series connected PV solar modules to an AC electricity, preferably in phase with an AC mains power. As will be described below, PV solar modules typically comprises a plurality of serially connected solar cells, typically comprising a plurality of groups of solar cells. In the event that certain cells are not generating electricity, and others in the serial connection are generating electricity, polarity of the non-generating cells, or group of cells, reverse. In order to prevent a damaging reverse potential, a bypass element is required such that in the event that the reverse potential exceeds a predetermined value a low resistance bypass path is provided.
Various hazardous conditions have been identified for which a safe mode of operation of a PV solar module is necessitated. These hazards can be separated into three categories.
The first category of hazard could be considered as being caused by the PV installation itself. For example, if a PV electrical current carrying conductive wire exhibits a poor connection, dangerous arcing may occur which can ignite a fire. As another example, if the electrical output of the PV system were to accidentally touch another electrical conductor of a different voltage potential, this can similarly ignite a fire. Attempts to mitigate the danger of fire causing arcing are addressed, or in the process of being addressed, by various electrical code setting organizations, such the National Fire Protection Association (NFPA) in the United States, which issues the National Electrical Code (NEC). Due to limited technology available at the time of code writing, not all PV arcing scenarios can be addressed. The 2011 NEC 690 addresses “series” faults, but does not address “parallel” PV-faults, primarily due to the paucity of commercially available solutions. In addition to the NFPA, the issue is being addressed by Underwriters Laboratory (UL) in the UL1699-PV ad hoc working committee, and by the International Electrotechnical Commission (IEC) in an IEC TC 82 committee.
The second category of hazard is electrocution hazard for emergency response crews. For example, if a structure which is in proximity to a PV installation is on fire, fire responders need an ability to remove all electrical hazards prior to entering the fire zone. At present, in DC-string inverter based PV systems, the fire responders can only disconnect the inverter or load. Unfortunately, the source of energy still remains and thus lethal levels, e.g. 600 Volts DC, of electrical energy are still present. It should be noted that the cause of the fire is irrelevant for this category of hazard. It may have been caused by the PV equipment or by other sources. Regardless, the electrocution risk to fire responders still needs to be addressed.
The third category of hazard occurs during the installation and/or maintenance of PV systems. For the most part, this hazard is mitigated by proper training of installation/service personnel. However, even with the best of training, accidents may occur. Because solar cells are energized whenever light is present, PV installation and servicing can not be guaranteed to occur in the absence of potentially lethal voltage unless a failsafe mechanism to prevent energy from being converted is available.
Various methods are being actively considered to resolve the issue, and a study of this matter has been presented at the Feb. 15, 2011 PV-Safety Conference held in San Francisco, Calif. Each of the various methods exhibit advantages and disadvantages, however the approach of arranging for a short circuit of the PV module seems to exhibit the best potential solution, since in such an approach: the inverter is free of input voltage; the DC main line is de-energized; the DC connection box is de-energized; the DC string is de-energized; the module itself is de-energized; existing inverters do not need to be redesigned; no additional line losses occur as a result of the safety mechanism; and automatic shut off in the event of an AC power failure can be arranged.
Unfortunately, the prior art does not teach any method of supporting such as safety mechanism, in particular since in the short circuit condition no power is available at the short circuited PV module to maintain the short circuit, or disable the short circuit responsive to an enablement signal.
In particular, a bypass transistor requires an energy source to allow it to be conductive. This statement would apply to enhancement mode MOS transistors as well as bipolar transistors. It does not apply to depletion mode transistors, but the complexity of turning these depletion mode transistors off in normal mode introduces an entirely new list of other considerations which adds to cost and complexity. In the event of a fire, no guarantee can be given that a reliable energy source for a signal to control these transistors will be present.